Really getting stumped with this one.

75V at each lug of the flipper in attract mode

50V at left (working) flipper lugs (blue wires) when button pressed
60V at right (dodgy) flipper lugs (blue wires) when button pressed

Swapped fliptronics board - no difference
Another new coil - no difference
Swapped and adjusted EOS - no difference

So to me it either has to be in the wiring somewhere (unlikely, due to voltage present), or is it a dodgy plunger link bushing or something.....